Known LED chips generate light in a specific region of the light spectrum. The light output from the LED may be, for example, blue, red or green, depending on the material composition of the LED. When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to convert the LED light output having a peak wavelength (the “primary light”) to light having a different peak wavelength (the “secondary light”) using luminescence/fluorescence.
The luminescent/fluorescence process involves absorbing the primary light by a wavelength-converting material such as a phosphor or mixture of phosphors thereby exciting the phosphor material, which emits the secondary light. The peak wavelength of the secondary light depends on the type of phosphor material, which can be chosen to provide secondary light having a particular peak wavelength. This process may be generally referred to as “wavelength conversion” and an LED combined with a wavelength-converting material, such as phosphor, to produce secondary light, may be described as a “phosphor-converted LED” or “wavelength-converted LED.”
In a known configuration, an LED die, such as a III-V nitride die, is positioned in a reflector cup package and a volume, conformal layer or thin film including wavelength-converting material is deposited directly on the surface of the die. In another known configuration, the wavelength-converting material may be provided in a solid, self-supporting flat plate, such as a ceramic plate, single crystal plate or thin film structure. The plate may be attached to the LED, e.g. by wafer bonding, sintering, gluing, etc. Such a plate may be referred to herein as a “wavelength-converting plate.” Another known approach is to provide the wavelength-converting material in a solid, self-supporting dome formed separately from the LED and attached thereto.
For design and/or cost reasons, a wavelength-converting plate configuration may sometimes be desirable. One drawback associated with using a wavelength-converting plate is that the wavelength-converting material at the periphery of the plate may receive less excitation from the primary light than the wavelength-converting material at the center of the plate. In addition, the path length of the primary light inside the said plate increases with changes in viewing angle from 0 degrees to the normal (typically defined at 90 degrees to the top surface of the wavelength-converting plate) to higher angles. These effects may contribute to different color light being emitted at angles close to the normal compared to the higher angles, which is known as color angular spread or color separation (ΔCx, ΔCy). For a well-known converter material, YAG:Ce, wavelength-converting plate and an InGaN LED chip, for example, bluer light may be emitted at angles near normal to the chip while yellower light may be emitted angles far from the normal.